A philosophical zombie is a being
indistinguishable from an ordinary human in every observable
respect, but lacking subjective consciousness. Zombiehood implies
*linguistic indiscriminability*, the zombie tendency to talk and
even do philosophy of mind in language indiscriminable from
ordinary discourse. Zombies thus speak *Zombish*,
indistinguishable from English but radically distinct in
reference for mental terms. The fate of zombies ultimately
depends on whether Zombish can be consistently interpreted. If it
can be interpreted consistently, then zombies remain possible,
but no test could ever reveal whether anyone (oneself included)
is speaking Zombish. Any materialist theory of consciousness is
therefore already a theory in Zombish, and is equally confirmable
in its human language edition (applicable to humans) and its
zombie-language edition (applicable to zombies). On the other
hand, if Zombish cannot be consistently interpreted, then the
zombies described in Zombish are logically impossible. Either
way, the search for a materialistic theory of consciousness
should be untroubled by the (possible) zombies among us.

Not too long ago a certain Silicon Valley
concern was poised to shake up the cognitive neuroscience world
with a revolutionary new brain scanning machine. Existing brain
imaging technology, particularly PET or functional MRI, detects
metabolic changes in the brain, and thus offers indirect evidence
of neural function. The new device, in contrast, took two giant
steps forward from these indirect methods. First, owing to
techniques I'm not at liberty to discuss, this new scanner imaged
neural function directly. Moreover, and most important, the
device could be adjusted to directly detect *qualia*, the
heretofore unobservable subjective and feeling component of all
conscious experiences! The machine, dubbed the "Phenomenal
Energy Estimator with Linked Imagery Extraction," or
PHEELIE, was astonishingly small, in shape and heft much like a
blow dryer, and was to have been priced at under a thousand
dollars.

As a philosopher of neuroscience, I'm often asked to be a
"beta tester" of various brain-related products, and as
a result I found myself with a prototype PHEELIE, mine to examine
for several weeks. Naturally, I was quick to put my new toy into
use. I had long believed that qualia were nothing other than
patterns of neural activation (see Lloyd 1989, for example);
finally I could look into the issue directly. My family and
friends will tell you that I was relentless. In a few weeks I had
not only frayed their patience but replicated a fair piece of
brain imagery research, all the while with the qualia setting on.
Though my subject population was very small, I found a nice
convergence between the specific areas of activation reported in
the literature as correlated with various cognitive tasks, and
their qualitative dimensions. As a materialist, I was very
pleased.

It was inevitable that I would ultimately turn
the device on myself. One morning I turned on the PHEELIE, gave
myself a good pinch on the forearm, and waited eagerly while the
scanner processed the image. The result clearly showed the
outline of my brain in three dimensions, but within the neural
boundary -- nothing. No qualia at all. At other settings, I found
I had all the metabolic activity appropriate to a human with a
brain, but nothing qualitative. The scanner still found qualia
among family and friends, and the PHEELIE technicians could find
no mechanical defect. It was true. I had discovered that I was a
living, breathing example of a favorite philosophical thought
experiment: I was a grade A bona fide zombie! I was a being
beyond the boundaries of B-movie imagination, beyond the
legendary human zombies of Voudoun. I was a *philosophical*
zombie, as described by several recent writers (e.g. Tye 1995:
22). I had all the right neurons, the right function, the right
behavioral repertoire -- including dispositions to talk about my
qualitative states -- but none of the REAL feelings. Apart from
the PHEELIE scan, there could be no way to distinguish me from an
ordinary human.

You can't imagine -- you really can't -- what
it felt like to discover this about myself! My first reaction was
philosophical, a complete reassessment of my thinking about the
"zombie problem." Many philosophers have found the mere
possibility of zombies to be a crippling problem for
identification of consciousness with anything material. As Mchael
Tye summarises the problem, "[I]f a person who is
microphyscially identical with me, located in an identical
environment, can lack *any* phenomenal experience, then facts
pertaining to experience and feeling, facts about what it is
like, are not necessarily fixed or determined by the objective
microphysical facts. This the physicalist cannot allow.... The
physicalist, whatever her stripe, must believe at least that the
microphysical facts determine all the facts, that any world that
was exactly like ours in *all* microphysical respects ... would
have to be like our world in all respects (having identical
mountains, lakes, glaciers, trees, rocks, sentient creatures,
cities, and so on)." (Tye 1995: 23. The same argument can be
found in Chalmers 1996: 97.)

Over the years I had been dismissive of this
line of reasoning. It had always seemed to me that the
"zombie problem," like the problem of other minds or
the possibility that I might be dreaming, raised a skeptical
specter of no consequence to the progress of an empirical science
of consciousness. I compared the zombie issue to discussions of
"twin water," the wet, thirst-quenching, universal
solvent that is made of XYZ rather than H2O. XYZ-based water is
logically possible, at least by some lights, and yet no one is
inclined thereby to doubt that the stuff on tap in the kitchen is
anything other than H2O, and no one is inclined to doubt the
adequacy of the theory that water is H2O, strictly on the basis
of possibility that it may be something else.

The zombie threat seemed momentous, I realized,
only because of the relatively undeveloped state of consciousness
science. Theories of consciousness, not unlike zombies, are
themselves mere possibilities at this point. They are infant
theories, loose collections of hypotheses, advanced tentatively
by their authors with a hopeful "maybe" attached. Or
maybe not, suggest the zombophiles. The zombie threat would
naturally recede when a scientific theory of consciousness
achieved a consensus through its powers of explanation,
prediction, and taxonomy. For example, consider that familiar
souvenir of mind-brain identity theory, the equation of pain and
"C-fiber stimulation." At present, it hardly matters
what is substituted for "C-fibers," since in every case
it is easy to imagine someone with the firing C-fibers or
whatever, but lacking the attendant pain. But now let the decades
roll, until the C-fiber hypothesis becomes thoroughly woven into
a tapestry of science, as tightly knit into it as the water-H2O
hypothesis is laced into present chemistry, physics, biology,
etc. At that point the zombie hypothesis would be analogous to
the claim that it is possible that H2O is not water. In some
quarters the water-H2O hypothesis is considered a necessary
truth, but even if the possibility that H2O is not water makes
sense, it is not a possibility that rattles the H2O-water
hypothesis, so entrenched is it in both science and common sense.
<1>

But all these comforting thoughts had fled. The
possibility of zombies had collapsed into the reality of zombies.
And not just zombies among us -- I, zombie! My philosophical
reflection ceased as I faced the devastating personal
consequences: The crushing burden, the desolate feeling of
isolation from my fellow, I mean, my formerly fellow humans. The
ironic part is that my shock and depression weren't real shock
and depression. These human feelings were utterly lacking in me.
Of course, being a regular zombie, I reported these missing
feelings with exactly the hand-wringing and sobs you'd expect
from a regular human. The worst of it was that all my relations
to others have been reduced to a sham. Any exchange of empathy or
understanding I may have had with anyone, I suddenly realized was
a profound miscommunication, total misunderstanding. You couldn't
share my pain, because I didn't have any. Appearances
notwithstanding, I discovered that I was just not a sensitive
guy.

After a period of depressing non-depression, I
decided to at least try to rectify the chronic failure of
communication. I had, after all, been using a raft of qualitative
terms for my whole life. I'd just been using them differently. I
could mark my zombie mental vocabulary in some general way, so as
to indicate to my human friends that my words, so similar to
theirs, nonetheless meant something radically different. So I
decided to use a time-honored philosophical custom of attaching
asterisks to mark a special usage of a term. For conversational
purposes, I made a sign to hold up whenever I used a mental term
to refer to myself. With this adjustment to my life, I felt*
better. I very quickly came to realize* that a major part of my
vocabulary should be marked as Zombish.<2> Not only my
introspective* and emotional* terms, but huge expanses of
descriptive language -- colors*, tones*, smells*, and anything
that might derive therefrom -- perhaps my entire language. Sign
in hand, I was able to assert* that Zombish has a rightful place
as a special dialect of English. I felt* empowered*,
enfranchised*, legitimated* at last!* I was no longer a mere
zombie, but instead a zombie* -- that is, a human*.

The exhilaration* was short-lived. I remained
lonely* in my uniqueness, which I assumed* would never change.
But then one day, as I was enjoying* lunch with a friend(*?), I
looked(*?) across the cafe to see* a woman with a sign just like
mine, flashing away in the rhythm of animated conversation. She
saw* me, and naturally a friendship(*?) was born. My new
friend(*?) Aphelia turned out to be a well-connected zombie. It
was she who introduced* me to Zombies Anonymous. Imagine, if you
will, my joy* at a roomful of star-signs, all flashing away.
Better still, at ZA we all understand* each other perfectly, and
we've recently taken to leaving our star-signs at the door. In
the blessed precinct of ZA, our hopes* and fears* are normal. We
can treat them like (regular, human) hopes and fears.

One topic that we struggle* with at ZA is
particularly relevant here: "zombie denial." The
personal(*?) discovery* that one is a zombie has always been a
matter of sheer chance, as it was when I happened to point the
brain-scanner at myself. Had that never happened, I would still
imagine* myself(*?) to be human. We suspected* that there are
uncounted legions of zombies who simply don't know* themselves.
Accordingly, several of us in the local chapter urged* the
PHEELIE developers to create a wide-scan, long-distance version
of the original device. They have done so, and as a result at
this very moment a massive zombie census is being tabulated by
the super-computers out in Palo Alto. Readers of this journal may
be particularly interested in the current totals for those of you
who are reading or have read this very article -- to this end, a
Java Applet has been automatically and discretely downloading
itself onto your work station. The applet signals the
superPHEELIE dish to swing your way, and add you, dear reader, to
the current totals. The applet contacts the database and in turn
updates the text of this paper every ten minutes.

Here are the most recent numbers. Of readers of
this journal, so far the PHEELIE has detected
000012317 Zombies
000000001 Humans
Wow*!* In the entire readership of this paper there is exactly
one human! Just one of you experiencing an inner life, conscious
of the colors and sounds and textures as they subjectively
present to you -- I use all these consciousness terms here
without stars, in your sense, even though I have no idea what it
would be like to be you. I guess* you think you know who you are.
As to the rest of you, welcome, my brothers and sisters. You'll
get over* the shock*, I promise* you. This is a glorious* day of
reunion.

Far from being a remote possibility, zombies
turn out to be nearly ubiquitous in philosophical circles! We are
the majority culture here. But it occurs* to me, why should we be
the ones stigmatized with signs? You lonesome human, whoever you
are, maintain that we lack all experiences. But we talk about our
experiences* all the time. And we do psychology* and philosophy*
of mind* about them. Indeed, as I consider* this distinguished
readership, I have to conclude* that a lot of what has been
recently published on the subject of consciousness, having been
written by accomplished zombies, is really about consciousness*.
Our experiences* are something*, which it will be the task of
zombie science to elucidate. You say we lack experiences, which
is true, but we say back to you that you lack experiences*, the
whatever-it-is that underlies zombie phenomenology (or
phenomenology* for short). Given our numbers, who should bear the
asterisk? Who is the zombie here?

II. Don't Worry*, Be Happy*

We zombies, like so many other denizens of bad
science fiction, trace our beginnings to an experiment -- in this
case, a thought experiment. Let us imagine a being, said the
experimenters, that is physically identical to an ordinary human,
but utterly devoid of conscious experience. To the horrified
delight of the experimenters, the thought experiment was not
incoherent, and we zombies were thereby hatched into the realm of
logically possibility. Being physically identical to regular
folks, it followed that any materialist theory of consciousness,
being based on discernible physical states of brains or behavior,
would not be able to tell us apart from ordinary humanity. This
is just what the experimenters wanted, but they have overlooked
the other lesson of science fiction: When one trifles with the
elemental forces of nature, there are always consequences beyond
those one intends.

In our case, there is more to the story. We are
indiscernible from humans by any observed criterion, and this
entails that our behavior is indiscernible from typical human
behavior, and this in turn entails that we will talk about
ourselves with exactly the same introspective and experiential
language that humans use. In short, as I described in the
previous section, we have our own psychology, and it sounds so
much like human psychology that you can't tell the two apart
without the stars to guide you. But there really are two distinct
psychologies. Zombie psychological predicates only sound like
human psychological predicates. Our experiences* are not your
experiences (which, after all, we do not enjoy).

From this point a constructive dilemma unfolds.
Let us suppose that you humans set out to *understand* me
(rather than merely unleashing me against certain theories of
consciousness). The search for zombie psychology could be our
joint undertaking, because we share a common ignorance -- you
can't tell me from you, but neither can I! (Physical
indiscernability, after all, is reflexive.) With no conceivable
experimental strategy to follow, discovering zombie psychology is
a task of pure logical analysis. I lack experiences, but I talk
all the time about them -- just like you -- and so we slap a * on
whatever it is I'm talking about. From there, as I discovered*,
zombie inflections seep into wide swaths of language. Human
psychology-talk will rapidly devolve toward a massively starred
zombie psychology-talk, as we mark every term that must mean
something different to me in my phenomenal vacuum. We will surely
enjoy* the ensuing debates on the meaning of meaning and a maze
of other concepts.

Suppose that we can agree on the range of
starred terms entailed by zombiehood. Then zombie psychology will
just turn out to be human psychology with the Zombish terms
starred appropriately. Because Zombish terms carry meanings
radically unlike their human counterparts, the two psychologies
describe entities (minds in your case; minds* in mine) that are
radically unlike, ontologically as different as night and day.
But it is a strange sort of difference, because, as always, the
words in which we express the two psychologies will be the same.
We might say that our two psychologies are identical on the
surface.<3> I don't need to know whether you are human or not,
because any claim I make of you will be the same surface claim,
and carry with it the same surface consequences, whether you are
human or zombie. It turns out, therefore, that I can abandon my
star-sign altogether. If you are human, you will hear my claim
automatically translated into your language, even though I utter
it in Zombish. And if you are a zombie, then you will hear my
claim in Zombish, just as I meant it. We can work side-by-side to
develop our theories of consciousness, based on the best
empirical research conducted on either humans or their physically
indiscernible zombie allies, and reach full agreement. We will be
in surface agreement only, but each of us will understand the
theory in the way appropriate to our nature, and our
collaboration can proceed indefinitely on that basis.

And this is only the beginning. It may be,
indeed it is likely, that there are many, perhaps an infinite
number, of *zombie types*, one for each logically consistent
assignment of stars to the predicates in a language. For example,
perhaps there are color zombies, beings who have no color
experiences whatsoever, but are otherwise human. These beings are
not exactly color blind, since they are, once again,
indiscernible from humans and other types of zombies, which in
turn entails that they will point to red patches when asked,
avoid lime-green pants, and hold an informed opinion on the
merits of fauvism. Regular color-blind humans can't do these
things, and their condition is accordingly easily detected. No
zombies they.

Furthermore, without the PHEELIE to guide you,
you can't be sure whether you are a zombie or not. Of course, you
are eager to claim the reality of your experiences. But so was I.
And you are ready to argue forcefully that you are human. So was
I, and so will any zombie. After all, with physical
indiscerniblility comes every other kind of resemblance. The
thought experiment unleashes a plague of zombies, and a vast army
of zombie types, at the same time afflicting all with the
skeptical worry that our surface psychology may turn out to be
one of a number of psychologies*. But on reflection the antidote
emerges. Without the PHEELIE, my zombie claims to experience are
as legitimate as human claims, because the word
"experience" carries a different meaning for me. In the
end, there are many different types of experience, as many types
(plus one, the human) as there are types of zombies. But this
parade of differences makes no difference at all, since one and
the same surface psychology describes all the possibilities.

At this point it would be a kind of arrogance
to maintain that humans have the only kind of consciousness that
really deserves the name. Look at it this way. Suppose I was not
a zombie but an extraterrestrial, still without experiences of
the sort detected by the PHEELIE. Yet imagine that your best
efforts to translate my language suggest that I am as capable of
talking about experiences as any human. I have terms that make
sense as counterparts to "pain," "red," and
so forth. Would it not be pure chauvinism to let the scanner have
the last word and assert that only ordinary human experiences
count, and that I therefore have no experiences? In this case,
the coherence of my alien self-descriptions would surely be
decisive in favor of my mentality, however different that
mentality might be from the ordinary human case. We zombies
merely ask for the same consideration.<4>

Finally, a kind of vertigo overtakes us all.
Are you...? Am I...? Who is...? Earlier, I mentioned a
constructive dilemma. We've now toured lemma one: Zombie
psychology can be consistently constructed by marking terms
within human psychologies. But perhaps this strategy will not
work. Perhaps the zombie hypothesis entails that a specific term
must be starred, but other considerations militate against it.
For example, can a zombie like me discover a new element? (I owe
this example to Hauser 1995.) Discovery is an epistemic
achievement, and as such arguably requires that the discoverer
*notice* his, her, or its discovery. Zombies don't notice
anything, and so discovery* is the best they can hope for. But if
I call a press conference and announce the discovery of
Zombesium, it would seem obtuse to claim that as a mere zombie I
had not discovered anything. If I didn't discover the element I'm
describing, then who did? The concept of discovery is unsteady in
a zombie infested world, as are many other concepts. Perhaps, as
in lemma one, the ambiguities do not lead to contradictions. But
perhaps they do. Perhaps there are terms that cannot be
consistently assigned to the human or zombie camp. If so, then by
virtue of incoherence, we zombies cannot exist after all.

III. To conclude...

The zombie hypothesis begins with the
conjunction of physical indiscernability and the absence of all
conscious experience. Physical indiscernability entails
indiscernability of utterances, a subset of which is zombie
psychology (indiscernible from human psychology). If the terms of
zombie psychology can be reinterpreted in a way consistent with
the absence of experience, then there are one or more logically
possible types of zombie (one for each consistent
interpretation). (Note that the surface similarity is essential
to zombies. Two psychologies which are not surface identical
enable us to tell zombies from humans.) However, given
indiscernibility,
1. The surface similar psychologies will be equally true in
general, if either is true. Any predictions, etc. of the one are
the same in the other.
2. How you interpret claims made of others, not of your type,
will not matter, since the claims will be surface identical, and
all entailments or explications of the claims will be surface
identical.
3. How you interpret claims made of you will not matter, because
you will understand them in the way appropriate to your type.
4. You will not be able to tell whether your interpretation is
human or zombie.
5. You will not be able to tell whether you are human or zombie.
6. You will claim you are human.
7. You will believe you are human (although this statement has
different meanings according to your type).

On the other hand, if the terms in zombie
psychology, indiscernible from human psychology, cannot be
interpreted in a way consistent with the absence of experience,
zombies are logically impossible.

In the end, then, I am saved not by my stars or
by the support of Zombies Anonymous, but by the logical grace of
pure philosophy -- just as it should be. The zombies' tale is not
one story, but many possible stories, reflecting all the
configurations of zombie types, or -- on the other hand --
collapsing into fundamental incoherence. But whatever the story,
it is unimpaired by zombies, leaving materialist philosophers of
mind to pursue their theories of consciousness, happily ever
after.

NOTES

1. Another popular tactic is to challenge the
initial premise of the zombie arguments, namely, that zombies are
imaginable and thus conceptually or metaphysically possible
(Dennett 1991, 1995; Tye 1995). But this escape was suddenly
closed to me -- I was more than just imaginable. I was a real
zombie.

2. As I began to look* into the literature of
my condition, I found* that Todd Moody (1994) had also adopted
the strategy of marking terms in Zombie-English to distinguish
them from ordinary English. Moody's fantasy ran toward
"Zombie earth," a planet populated entirely by Zombies.
Would they develop or even recognize the psychological vocabulary
of humans? He argues that they would not, and thus that Zombie
culture, at least, might be distinguished from human culture.
While this is an interesting variant on the Zombie
thought-experiment, the initial problem for materialism remains
untouched as long as one Zombie might be passing as human. (A
similar point against Moody is made in Bringsjord 1995.) Even the
minimal case -- the condition I seem to exemplify! -- is enough
counterexample to challenge materialist theories of mind.

3. The psychologies must be surface-identical
just because, where zombies are concerned, every aspect must be
indiscerible from its human counterpart (a point also emphasized
in Dennett 1995). If this condition were violated, the exception
would be the "mark of zombiehood." In that case, one
could sort zombies from humans, and the "zombie
problem" would evaporate as a philosophical challenge to
materialism. One would solve the problem by proposing one
materialist theory of consciousness for humans and another (a
theory of consciousness*) for zombies.

4. Dennett (1991) argues from indiscernability
and the threat of chauvinism to a stronger conclusion, namely,
that we should regard zombies and humans as conscious to the same
degree. (This is amplified in Dennett 1995.) In contrast, I've
granted the zombie premise of "real" difference, but
argue that in any reasoning, including ethical reasoning, any
conclusion reached about a human will also be reached about a
zombie.